1. Field of the Invention
The invention relates to a pulley system for use in a transmission, particularly a constant variable transmission. The invention more specifically relates to the pulley flange, also known as a sheave, and belt in such a pulley system.
2. Description of the Related Art
Variable ratio drive pulleys, which are known, are generally employed to transmit a drive force from an engine via a belt transmission to a driving mechanism. Typical pulley systems comprise a pair of opposed frusto-conical flanges or sheaves mounted on the output shaft of the engine and arranged to apply a drive to an endless flexible belt positioned between the flanges. Both flanges rotate with the output shaft, one flange being axially fixed, and the other flange being axially shiftable towards and away from the fixed flange. A mechanism is generally provided to urge the movable flange away from the fixed flange so that at idling or low engine speeds there is no driving contact between the flanges and the belt, and no torque is transmitted.
To effect driving engagement between the flanges and the belt, such pulleys include an actuating mechanism such as centrifugally actuated weights, levers and the like that respond to increasing engine speeds to apply an axial force to the movable pulley flange to shift it against the spring towards the fixed flange and drivingly engage the belt between the flanges. The drive ratio is determined by the diameter of the belt path where it is engaged by the flanges, and varies with shifting of the movable flange towards or away from the fixed flange.
An example of a variable ratio drive pulley is described in U.S. Pat. No. 4,575,363, which is hereby incorporated by reference into this disclosure in its entirety.
In a constant variable transmission, the belt rides the inner surface of the sheave pulley and is thus subjected to stress during operation. Conventional sheave pulleys have surfaces that are conical and incline at a constant angle. In such constant variable transmissions, belt life has been a problem, especially for consumers, because the stress experienced by the belts can cause failure. Other pulley systems use curved sheave surfaces, such as U.S. Pat. No. 4,631,042, in which the sheave surface curves based on a constant radius R to reduce the pressure at contact points between the belt and the sheave.
One way of strengthening the belt to alleviate problems associated with stress is to thicken the belt. However, wider belts create higher power losses because the inside of the belt deforms. Also, the centrifugal force on thicker belts is higher, and more heat is generated during operation thus decreasing the efficiency of the belt. Tension can be varied in the belt by adjusting the angle of inclination of the inner surface of the sheave. Thus, some devices use a smaller belt while still addressing the problems associated with stress by providing sheave pulleys with inner surfaces having variable sheave angles.
U.S. Pat. No. 3,548,670 is an example of a pulley system in which the sheave surface has several discreet sections, each having a different angle of inclination with a step therebetween, to increase the output range by increasing the adjusting range by providing different cone angles. The disadvantage in this case is the change in angle of inclination creates a point or an edge which increases the stress on the belt when the belt is rotating at the radius of the edge.
Another common problem for such pulley systems is belt misfit, often due to distortion of the belt from bending it on the pulley. A belt""s bending distortion varies with the radius to which it is bent and also varies with the belt material. The distortion can result in power loss. To enhance efficiency, the belt should match the pulley as closely as possible, which is difficult to achieve when the belt is distorted.
As is well known in the automobile industry, constant variable transmissions are used in combination with steel belts which do not distort due to the pressure from the sheaves, this permits the sheave angles to have a frusto-conical shape and obtain a high efficiency output. The benefits of steel belts however would be difficult to obtain in a high-rev engine such as the two-stroke engines used in a snowmobile. In order to use a CVT with a steel belt in a snowmobile, an additional gear box would have to be used to decrease the RPM""s of the engine to that suitable for use with a CVT using a steel belt.
Accordingly, there is a need for a pulley system that provides adjustability and also manages belt stress to enhance belt life. There is also a need to provide a system that is designed to match the pulley to the belt to increase efficiency.
An aspect of embodiments of the invention is to provide a pulley system in which the sheave surface is progressively angled and traverses an arc that has a variable radius.
Another aspect of embodiments of the invention is to provide a sheave surface that corresponds to the belt edge. With close correlation between the side angle of the belt and the sheave angle, the power loss during clutching of the constant variable transmission is reduced.
A further aspect of embodiments of the invention is to provide an arrangement that can enhance belt life. A pulley system in accordance with embodiments of the invention can increase belt life by reducing stress experienced by the belt since the sheave surface can closely match the profile of the belt.
An additional aspect of embodiments of the invention is to provide an arrangement that can reduce power loss during operation.
These and other aspects of the invention can be realized by providing a variable drive pulley assembly for use in a transmission including a drive shaft rotatable about an axis of rotation, a fixed flange supported for rotation on the drive shaft, and a movable flange supported for rotation on the drive shaft and axially movable on the drive shaft toward and away from the fixed flange. The fixed flange has a first inner surface and the movable flange has a second inner surface that faces the first inner surface. A biasing mechanism is disposed adjacent to the movable flange that urges the movable flange away from the fixed flange. A torque transmitting mechanism is coupled to the movable flange and transmits driving torque from the drive shaft to the movable flange to move the movable flange toward the fixed flange. At least one of the first inner surface and the second inner surface is disposed at an angle inclined from a reference line perpendicular to the axis of rotation. The angle progressively varies across the at least one inner surface and follows an arc having a variable radius.
The variable drive pulley can further include a drive belt disposed between the fixed flange and the movable flange, wherein the drive belt is slidable along the inner surfaces of the fixed flange and the movable flange. The belt can have an outer surface that is complementary to the at least one inner surface of the respective fixed flange and movable flange.
The invention also provides a constant variable transmission including a drive shaft rotatable about an axis of rotation, a pulley sheave assembly mounted on the drive shaft, and a belt coupled to a drive mechanism and disposed around the drive shaft and the pulley sheave assembly. The pulley sheave assembly includes a fixed flange supported for rotation on the drive shaft and a movable flange supported for rotation on the drive shaft and axially movable on the drive shaft toward and away from the fixed flange. The fixed flange has a first inner surface and the movable flange has a second inner surface that faces the first inner surface. At least one of the first inner surface and the second inner surface is disposed at a progressively variable angle with respect to a reference line that is perpendicular to the axis of rotation. A biasing mechanism is disposed adjacent to the movable flange that urges the movable flange away from the fixed flange. A torque transmitting mechanism is coupled to the movable flange and transmits driving torque from the drive shaft to the movable flange to move the movable flange toward the fixed flange. The movable flange engages the belt and the belt moves across the first and second inner surfaces based on transmitted torque. At least one of the first inner surface and the second inner surface is disposed at an angle inclined from a reference line perpendicular to the axis of rotation, wherein the angle progressively varies across the at least one inner surface and follows an arc having a variable radius.
The invention also provides a method of transmitting drive force from an engine to a drive assembly while varying a drive ratio between a drive pulley assembly coupled to the engine and a transmission belt coupled to the drive assembly. The method includes providing a drive force to a rotatable shaft, transferring drive torque from the rotatable shaft to a drive pulley assembly having a fixed flange and a movable flange and moving the movable flange toward the fixed flange, engaging a transmission belt between inner facing radial surfaces of the fixed flange and the movable flange, and varying the drive ratio between the belt and the drive pulley assembly by sliding the belt across the inner surfaces of the fixed flange and the movable flange. The inner surfaces are disposed at a progressively variable angle with respect to a reference line perpendicular to the axis of rotation of the rotatable shaft and follow an arc having a variable radius.
The pulley system in accordance with the invention can be used in any suitable transmission, especially a constant variable transmission. The pulley is well suited for use in vehicles, including but not limited to snowmobiles, all terrain vehicles (ATVs), tractors, scooters, agricultural vehicles, automobiles or trucks.
These aspects and other aspects, objects and advantages of the invention will become apparent from the following description and drawings that illustrate preferred embodiments of the invention.